1. Field of the Invention
The present invention relates to a steering system in which an operation of toe angles of rear wheels is controlled based on a turning angle of front wheels and a vehicle speed, and particularly to a steering system in combination with an electric power steering device which assists steering wheel turn of the front wheels.
2. Description of the Related Art
An electric power steering device is a device in which an electric motor generates an auxiliary torque in accordance with a magnitude of a steering torque, and the auxiliary torque is transmitted to a steering unit, to thereby reduce a steering effort required by a driver for steering. In Japanese patent application examined publication No. H6-47388 (FIG. 2) (hereinbelow, referred to as “Patent Document 1”), there is disclosed an all-wheel independent steering device in which operation of all running wheels are individually controlled based on an operation angle of a steering wheel and a vehicle speed. Also in Japanese patent application unexamined specification No. 2005-199955 (FIG. 6) (hereinbelow, referred to as “Patent Document 2”), there is disclosed a technique in a steering device of a steer-by-wire type vehicle for informing an anomaly to the driver, in which, when an abnormal state (failure) associated with the steering wheel turn occurred, an electronic control unit selects a map that gives a larger (or “heavier”) steering torque, based on a conversion table including a relationship between turning angle and steering torque, to thereby make a steering torque in response to the steer wheel operation of the driver become larger as compared with a driving in a normal state.
There is also disclosed a technique in which a base signal (auxiliary torque) based on steering torque and vehicle speed is compensated with inertia and damping (viscosity) in the steering unit, and an electric motor is controlled using the compensated signal as a target current (see Japanese patent application unexamined specification Nos. 2002-59855 (FIG. 2) and 2000-177615 (FIG. 2) (hereinbelow, referred to as “Patent Document 3” and “Patent Document 4”, respectively)).
In Patent Document 3, properties including a base signal, damping and inertia are computed using a base table, a damper table and an inertia table which substantially has a differential property, respectively. Herein, setting of each table, which includes functions of steering torque, vehicle speed and electric motor angular velocity, will be discussed. The base table is set in such a manner that a driver is provided with road information and a steady responsive feeling from a steering torque, in accordance with an increase in the vehicle speed, and thus it is required that a gain be made lower when the vehicle speed is higher, and that a dead zone is set larger for giving a larger manual steering zone. The base table is also set so as to give an excellent steering feeling, and therefore, it is required that a response lag, which may otherwise be caused by electric motor inertia, viscosity or the like, be reduced by using the inertia table.
Since the road reaction is reduced when the vehicle runs at a higher speed, the damping control introduces inhibitory control to the motion of the electric motor at higher rotational speed range, to thereby impart stability to steering feeling. Therefore, in the damping control, a target current is corrected (damped) with a compensation value corresponding to damping gain. In the vehicle having a toe angle changer capable of turning the rear wheel, the toe angle of the rear wheels are also taken into consideration upon the damping control.
In the vehicle steering system for controlling vehicle properties by performing drive control of toe angle of rear wheels (RTC: Rear Toe Control), drive control by RTC may be failed (drive control by RTC may be in an abnormal state). In this case, it becomes impossible to control the rear wheels, and the toe angle of rear wheels become fixed, leading to change in a yaw rate gain of the vehicle. Especially, in the case of the anomaly, such as toe-out or antiphase failure, under a high yaw rate gain, the vehicle tends to be unstable, and thus stabilizing the vehicle body is highly demanded.
There is disclosed a technique for stabilizing a vehicle in which vehicle properties, such as yaw rate, are fed back to the system, and a control with an electric power steering device is performed using a reaction force based on the feedback (see, for example, Japanese patent application unexamined specification No. 2003-81117 (hereinbelow, referred to as “Patent Document 5”)).
However, in the conventional techniques, such as those disclosed in Patent Document 5 which uses the feedback of the yaw rate or the like, a reaction force value based on the feedback is determined simply for each vehicle speed. As a result, the reaction force value based on the feedback, which is supposed to be determined in accordance with the vehicle properties, cannot be properly altered. When the vehicle properties has changed due to failure of the drive control by RTC, the vehicle tends to be unstable, but the conventional technique cannot respond to the change, leading to poor stabilization of the vehicle.
In addition, when the toe angle changer is in an abnormal state, for example, when the toe angle changer is locked with the wheels toed out or with an antiphase control state, and the vehicle runs at a higher speed, the vehicle behavior becomes unstable.
Moreover, if the locking as described above occurs when the vehicle runs at a low speed, the driver may not sense anomaly (may feel as usual), due to the assisting effect by auxiliary torque in the electric power steering device.
In order to solve this problem, it may be natural to come up with the proposal that, when the toe angle changer is in an abnormal state, the steering torque is made larger than usual as described in Patent Document 2, and a means to notify the anomaly to the driver is further provided. However, a sudden change in the steering torque in response to the anomaly detection of the toe angle changer will give an awkward feeling to the driver.
Also in the techniques disclosed in Patent Documents 3 and 4, when the toe angle changer is in an abnormal state, the toe angles of rear wheels are fixed and the reaction force properties are changed, which may give unnatural steering feeling to the driver.
In the technique disclosed in Patent Document 1, when the power steering device is in an abnormal state, such as no auxiliary torque is output from the power steering device, the operational reaction force on the steering wheel becomes larger than that in the normal state, leading to delay in steering wheel operation by the driver. If the toe angle changer is normally acted under such a condition, the turnability is not reduced in response to the reduced steerability, leading to a discomfort to the driver.
Therefore, it would be desirable to provide a steering system that solves above-mentioned problems in vehicle stability and steerability associated with anomaly in the relationship between the rear wheel toe angle changer and the power steering, by performing a control in the combination of the power steering and the rear wheel toe angle changer.